In general, liquid fuels produced from petroleum (e.g. gasoline, diesel, kerosene) have chemical additives dissolved therein to enhance certain properties (e.g. to facilitate handling or to improve performance). The proportion of such additives may be very small (e.g. less than 100 ppm) yet may need to be controlled carefully to ensure the fuel exhibits the desired properties.
Water is not usually used as an additive, but is normally present in small amounts in liquid fuel. Water can exist in the fuel in three states: dissolved in the fuel, in a separate liquid phase (known as free water), or in a fuel-water emulsion. The existence of free water or a fuel-water emulsion in use can cause problems, particularly in aerospace applications where the chance of the water freezing is increased. It is desirable to monitor the amount of water dissolved in fuel, e.g. a measure of how close the fuel is to saturation, to provide an indication of the risk of free water or a fuel-water emulsion occurring.
Water-in-fuel sensors for detecting undissolved water have previously been proposed. For example, U.S. Pat. No. 4,638,305 describes apparatus for detecting free water in fuel by sensing its resistive properties. Similarly, U.S. Pat. No. 5,642,098 describes detecting water in an oil-water emulsion by monitoring the dielectric properties of the emulsion. Optical techniques have also been proposed. For example, U.S. Pat. No. 5,121,986 describes immersing a plurality of optical guides into fuel, each guide having a different refractive index such that the water content could be sensed by comparing or correlating changes in the intensity of light propagating through the guides.
More recently, a technique of detecting water dissolved in kerosene using a water-sensitive coating on a silica-based optical fibre containing a long period grating (LPG) has been demonstrated [1]. Here the presence of water causes a variation in the refractive index of the coating, whereby the LPG optical fibre operates as a spectral loss element. The central wavelength of the attenuation band in a transmitted spectrum is monitored to detect the amount of water dissolved in the fuel.